This invention relates to a method and apparatus for refining silicon containing impurity elements such as metallurgical grade silicon using an electron beam and obtaining high purity silicon for use in solar cells or other semiconductor devices, for example.
Metallurgical grade silicon, which is a starting material for manufacture of high purity silicon, is obtained by reducing quartzite. The degree of purity of metallurgical grade silicon is too low for it to be used as is for the manufacture of semiconductors, and it cannot even be used without further refining as silicon for the manufacture of solar cells (referred to as solar grade silicon), which has a lower purity than silicon for manufacture of semiconductors.
The primary impurities in metallurgical grade silicon are metals. These impurities can be removed by the physical method of directional solidification. The remaining impurities are primarily boron and phosphorus, which cannot be removed so easily. Therefore, in the past, metallurgical grade silicon has typically been refined by chemical methods, such as the Siemens method, which forms extremely high purity silicon referred to as polycrystalline silicon or polysilicon. Polysilicon has a sufficient purity for use in semiconductors, but such a high purity is not required for solar grade silicon. Accordingly, there is a desire for a simpler method of refining silicon to obtain higher grades of silicon, such as solar grade silicon.
Japanese Published Unexamined Patent Application Hei 10-245216 discloses a method and apparatus for producing solar grade silicon using a metallurgical process rather than a chemical process. The method disclosed in that patent document takes into consideration three types of impurity elements contained in metallurgical grade silicon, i.e., impurity elements such as phosphorus and antimony which can be evaporated by irradiation with an electron beam in a vacuum and removed, impurity elements such as boron and carbon which can be evaporated and removed by oxidation with an oxidizing gas (such as water vapor) which is added to a high temperature plasma gas stream, and metallic impurity elements which can be removed by directional solidification. In the method disclosed in that document, these different groups of impurity elements are removed by separate procedures.
In the conventional method for refining silicon disclosed in the above-described patent document, it is necessary to carry out removal of impurity elements such as phosphorus and antimony and removal of impurity elements such as boron and carbon in separate atmospheres. Namely, particles of metallurgical grade silicon are irradiated and melted with an electron beam in a first vacuum chamber to remove impurity elements such as phosphorus and antimony, lumps of silicon resulting when the molten silicon solidifies are pulverized to form silicon particles, the silicon particles are moved to a second vacuum chamber, the silicon particles are melted by a high temperature plasma gas stream, and impurity elements such as boron and carbon are oxidized and removed by an oxidizing gas added to the plasma gas stream.
Accordingly, in the above-described conventional method for refining silicon, two vacuum chambers are necessary as well as an electron gun for melting silicon and an apparatus for generating a plasma gas. As a result, a refining apparatus used for that method becomes extremely large and expensive. In addition, between the two removal processes, it is necessary to grind lumps of silicon and transport the ground silicon between the two vacuum chambers, so the efficiency of that method is poor.